WASHINGTON - Vice President Joe Biden, a man with nearly four decades of experience in politics, has not taken lightly his preparations for his debate against GOP vice presidential candidate Paul Ryan, holding practice sessions and perusing briefing books.

In the wake of President Obama's panned performance in his first debate against Mitt Romney, the stakes for Biden are higher than ever. In Thursday's debate he must not only avoid making any gaffes but also try to puncture his opponent's arguments on taxes, entitlement reform and deficit reduction, something Obama was criticized for failing to do.

The pressure on Ryan has risen as well. Romney greatly exceeded expectations, appearing both presidential and in command of the debate stage. Ryan must prove that he is potential presidential material -- while also defending the numbers that Romney put forth last week, especially on tax cuts.

Both sides are offering sky-high predictions for the other team. "There's a lot on the line," said Rep. Jason Chaffetz of Utah, a chief Romney-Ryan ally. "President Obama failed to defend his record and could not articulate a vision for the future. So I think that challenge now falls to Vice President Biden."

Bill Burton, cofounder of the Democratic super PAC Priorities USA, argued that the burden is actually on Ryan. "Ryan is going to face pressure to explain some of the dishonest claims he and Romney have been making, especially on things like Medicare, taxes and the auto industry," he said.

One point on which both sides appear to agree is that Biden is likely to be more aggressive in his faceoff against Ryan than Obama was debating Romney.

Mary Altaffer, Associated Press

Paul Ryan

Speaking with reporters last Thursday before a Cedar Rapids, Iowa, rally at which he sharply criticized both Romney and Ryan, Biden previewed the debate by noting that he's been "studying up on Congressman Ryan's positions on the issues."

"... I just want to make sure that when I say these things, I don't hear the congressman say 'No, no, no, I don't have that position,' or, 'That's not the governor's position,'" Biden said.

Ryan, for his part, said that he believes Biden will be an aggressive opponent. "I expect the vice president to come at me like a cannonball," Ryan told the conservative Weekly Standard Thursday. "He'll be in full attack mode, and I don't think he'll let any inconvenient facts get in his way."

Lots of debate prep

Both White House contenders have been deeply engaged in debate preparations even as they maintain busy schedules. For Biden, that has meant squeezing in time for debate prep while visiting with his family in Delaware, as he did one weekend earlier this month. For Ryan, it's meant practice sessions in Washington, as well as in his hometown of Janesville, Wis.

Biden, who kicks off an intensive "debate camp" this week in Wilmington, Del., so far has engaged in two mock debates with Rep. Chris Van Hollen of Maryland, who is playing the role of Ryan in practice sessions.

Ryan on Friday wrapped up a three-day debate camp in southwest Virginia; prior to that, he had held three mock debates with former U.S. Solicitor General Ted Olson, who is playing the role of Biden. Kerry Healey, who served as Massachusetts's lieutenant governor when Romney was governor, has been playing the role of moderator Martha Raddatz of ABC.

Then, there's the matter of the age and stature gap between the two candidates. Biden, 69, has run for president twice and served in the Senate for 36 years. Ryan, 42, has served in the House for 14 years and will make his first appearance on the national debate stage on Thursday in Danville, Ky.

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